Sputtering is the physical ejection of material from a target as a result of ion bombardment of the target. The ions are usually created by collisions between gas atoms and electrons in a glow discharge. The ions are accelerated into the target cathode by an electric field. A substrate is placed in a suitable location so that it intercepts a portion of the ejected atoms. Thus, a coating is deposited on the surface of the substrate.
In an endeavor to attain increased deposition rates, magnetically enhanced targets have been used. In a planar magnetron, the cathode includes an array of permanent magnets arranged in a closed loop and mounted in a fixed position in relation to the flat target plate. Thus, the magnetic field is caused to travel in a closed loop, commonly referred to as a "race track", which establishes the path or region along which sputtering or erosion of the target material takes place. In a magnetron cathode, a magnetic field confines the glow discharge plasma and increases the length of the path of electrons moving under the influence of the electric field. This results in an increase in the gas atom-electron collision probability. This leads to a much higher sputtering rate than that which is obtained without the use of magnetic confinement. Further, the sputtering process can be accomplished at a much lower gas pressure.
Reactive sputtering techniques have been employed to form metal-oxide films. In reactive sputtering, a reactant gas forms a compound with the material which is sputtered from the target plate. When the target plate is a metal, and the reactive gas is oxygen, a metal oxide film is formed on the surface of the substrate.
With the advent of cylindrical magnetron, further improvements in reactive sputtering have been demonstrated. For instance, a method for depositing metal oxide films, particularly dielectric materials such as silicon dioxide, employs a rotatable cylindrical magnetron in DC reactive sputtering. See Wolfe et al., U.S. Pat. No. 5,047,131, issued Sept. 10, 1991. One advantage of rotatable cylindrical magnetrons is the self-cleaning effect which eliminates much of the arcing problem brought about by dielectric material buildup. However, even with cylindrical magnetrons, the reactive sputtering rates for some metal oxides, such as titanium oxide, are so low that reactive sputtering is prohibitively expensive.
Another means of enhancing reactive sputtering in magnetron systems is to modify the flux of reactant gas to the target relative to the flux to the substrate. In this regard, a baffle is positioned between the target and the substrate with the reactive and inert gas outlets strategically located within the vacuum chamber. See Wirz, U.S. Pat. No. 4,988,422, issued Jan. 29, 1991. While such modified magnetron systems have achieved some degree of success, problems remain. For instance, debris often accumulates along the edges of the baffle apertures which eventually falls onto the substrate. In addition, conventional baffle systems are difficult to scale-up to accommodate larger substrates. Finally, the reactive sputtering rates for many important metal oxides, including titanium oxide, remain low.